This invention relates to a detector for use in various optical measuring instruments such as a double-beam spectrophotometer or an instrument for measuring the light reflectivity of a sample. More particularly, the invention relates to a detector which comprises a single photoelectric element and a beam mixer disposed adjacent the photosensitive surface of the element for alternately directing two beams of light, one from a sample being measured and the other for use as a reference, to the photosensitive surface of the single photoelectic element.
In a light reflectivity measuring instrument, the light beam emerging from a monochromator is divided by a beam splitter into two beams, one of which is used as the reference beam while the other beam is projected onto a sample in the form of, for example, a sheet for measurement of the light reflected by the sample.
For measurement of the reflectivity of light by the sample if two different photoelectric elements are used separately to detect the reference and sample beams, errors will be caused in the result of measurement due to drift and/or difference in sensitivity characteristic between the two elements.
To overcome the difficulty the two optical paths of the reference and sample beams may be combined into a single path so that the two beams are projected onto a single photoelectric element. Such combination, however, requires several mirrors to be added to the optical system so that the instrument as a whole becomes larger-sized and the inner structure more complex.
The same problem is encountered in a double-beam spectrophotometer.